In the field of gas concentrators it is known to use, for example, zeolite to adsorb adsorbate in a gas concentrator. The use of zeolite herein is intended to be exemplary. It would be known to one skilled in the art to tailor the use of a specific adsorbent, whether a particular type of zeolite or other adsorbent. As is known in the prior art, zeolite consists of molecular sized polyhedral cages. Oxygen and nitrogen molecules (for example) can access the inside of these cages through holes in the crystalline structure. The crystalline structure contains cations. Gas adsorption occurs when molecules respond to the forces of physisorption, which fall into two main categories: van der Waals (or dispersion) forces and electrostatic forces. The latter are prominent in attracting the gas molecules to these cations. Nitrogen molecules, for example, bind more strongly to the zeolite cations than do oxygen molecules. As a result, if a mixture of nitrogen and oxygen, such as found in atmospheric air, is pressurized into a chamber full of zeolite particles, nitrogen will adsorb into the zeolite particles more readily than does oxygen. There will be a higher concentration of oxygen in the empty space between the zeolite particles, (hereinafter referred to as zeolite void space), than there was in the original gas mixture.
Adsorption processes commonly employ fixed beds of adsorbent particles. These fixed beds are normally within a vessel, which when cylindrical is called a column. The adsorption process frequently operates cyclically in which uptake and release (regeneration) occur repeatedly. The adsorbent particles may be granules, beads, or pellets, as well as other diverse shapes. Being fixed implies that the adsorbent is generally stationary (held in place by gravity or other mechanical forces) while the fluid being treated flows between the adsorbent particles. Depending on the geometry, the flow direction may either be axial or radial, or in some other consistent direction, during a particular step in an adsorption cycle. Commonly, the flow direction reverses from one step in an adsorption cycle to the next, i.e., from upwards to downwards or from radially outwards to radially inwards, depending on the mechanical layout of the adsorption vessel.
In particular, LiX and LiLSX zeolites as adsorbents for nitrogen and oxygen, which are sensitive to moisture and carbon dioxide, as shown by Dr. J. C. Santos (Ph.D. dissertation in chemical engineering, University of Porto, Portugal, 2005). Dr. Santos conducted tests to determine the causes of loss of capacity of the adsorbents after being used in a pressure swing adsorption unit. H is study involved exposing the adsorbent to water vapor, or carbon dioxide, and both simultaneously. After regenerating it, he determined a nitrogen adsorption equilibrium point to assess the loss of efficiency.
U.S. Pat. No. 6,471,748 to Ackley discloses the removal of contaminants of air, typically water and carbon dioxide, in a pretreatment stage at the feed end of the adsorbent bed, by zeolites, activated alumina, activated carbon and/or silica gel. Highly-exchanged LiX zeolites are taught to be useful in the main stage of the disclosed process. Likewise, Rege, et al., “Limits for Air Separation by Adsorption with LiX Zeolite” Ind. Eng. Chem. Res. (1997), vol 36, pp. 5358-5365, teaches the use of a pretreatment bed to remove water and carbon dioxide from the feed gas before it enters the main LiX bed. Notaro, et al., in U.S. Pat. No. 5,810,909, also describe using layers of adsorbents to prevent deactivation of lithium-exchanged X zeolite. U.S. Pat. No. 6,824,590, to Dee, et al., shows different configurations of pretreatment layers, intended to protect lithium-exchanged X zeolite from deactivation by moisture or carbon dioxide.
Finally, U.S. Pat. No. 7,608,133 shows an adsorbent process for separating CO2 from an air stream using LiX adsorbents where a passive, check valve is used between the LiX zeolite adsorbent bed and the LiX zeolite column.
None of this prior art suggests physically isolating the pretreatment layer(s) from the LiX or LiLSX layer(s) by means of a controllable (switchable) valve.
U.S. Pat. No. 7,491,261 discloses an improved sieve bed design to manage breakthrough and the mass transfer zone by way of volumetric division. Fractionation of air to recover a highly enriched oxygen fraction is an exemplary use of the '261 technology. An empty space in the product end is separated from adsorbent-filled sieve space in the feed end by a mid-diffuser plate. The ratio of the empty product end void space to the adsorbent filled sieve space within a sieve bed may be determined by the relative percentages of the gases to be separated and the bulk loading factor of the molecular sieve. A product end void space of the correct volume may ensure the maximum volume of nitrogen has been adsorbed before breakthrough occurs. In operation, pressure in the sieve bed empty space and sieve filled space may be equal at any instant. This contains breakthrough to the location of the mid-diffuser plate.